The present invention relates, generally, to the measurement of an ionization current of a spark plug, in particular of resonant structure type spark plugs, used in ignition systems for motor vehicles.
The invention is particularly suitable for so-called “radiofrequency” ignition systems comprising multiple-spark type resonant structure spark plugs or BME.
These ignition systems using alternating currents are described, for example, in the French patent applications FR 2 859 830, FR 2 589 869 and FR 2 859 831, in the name of the Applicant.
At the end of the compression cycle, the spark plug is responsible for forming an electrical arc, the energy of which is sufficient to trigger the process of ignition of the gaseous mixture contained in the combustion chamber of the engine.
This electrical arc corresponds to the ionization of the gaseous mixture located between the electrodes of the spark plug, respectively a positive central electrode and a ground electrode.
However, on combustion of the mixture, after the spark has been generated by the spark plug, the flame edge can be propagated. Its blast can push back a portion of the mixture against the walls of the cylinder and the top of the piston.
The rise in pressure and temperature is so great that the fuel can remain jammed against the walls, reach its self-ignition point and then ignite in several places.
The result of this is microexplosions producing vibrations in the acoustic domain (between approximately 5 and 10 kHz). These vibrations are very strong and can rapidly create hot spots which further accentuate the problem. The accumulation of microexplosions will break off or melt a small quantity of metal on top of the piston and/or on the walls of the cylinder, which can, after some time, result in the destruction of the piston and of the walls of the cylinder.
It is possible to detect the appearance of these knock phenomena, by measuring the ionization current, that is, the current passing through the spark plug. In practice, an ionization current appears through the spark plug as if a resistor were temporarily placed at the terminals of the electrodes (according to a first approximation).
For this, the measurement means or sensors must be able to operate in a very narrow bandwidth, for example of around 7 kHz.